And here is Wikipedia's conversation about it.
If you read down a ways in the Wikipedia article, you'll see a discussion of how the mandala is used in different religions. It was interesting to see that it's present in religions aside from just Buddhism and Hinduism.

I also found a nice, simple description of a mandala: "The Mandala acts as a support for anyone in meditation. The Mandala itself is often illustrated as a palace with four gates, facing the four corners of the Earth. It is a symbol that represents wholeness, and can be seen as a model for the structure of life itself."